The present invention relates to a recording medium that can be magnetized vertically, having a plate shaped carrier body of nonmagnetic material, the at least one flat side of which is provided with a thin nonmagnetic intermediate layer having a very smooth surface, to which at least one magnetic recording layer of a material is applied, the axis of easy magnetization of which points in a direction perpendicular to the surface of the medium. Such a recording medium is known, for instance, from German Pat. No. 29 23 682. The invention further relates to methods for the manufacture of such recording media.
The principle of vertical magnetization for storing information in special recording media is generally known. (See, for instance, "IEEE Transactions on Magnetics", vol. MAG-16, no. 1, January 1980, pages 71 to 76, or U.S. Pat. No. 4,287,544). The recording media which are to be provided for this principle, frequently also called vertical magnetization, can also be present, for instance, in the form of rigid magnetic recording discs. Such a recording medium comprises at least one magnetizable recording layer of predetermined thickness which contains a magnetically anisotropic material, especially of a CoCr alloy. The axis of the so-called easy magnetization of this layer points perpendicularly to the surface of the medium. By means of special magnetic heads, the individual pieces of information can then be written along a track as bits in successive sections, also called cells or blocks, by appropriate magnetization of the recording layer. The bits have a predetermined dimension, also called wavelength, in the longitudinal direction of the track. This dimension can be substantially smaller than the limit given by the demagnetization with storage according to the known principle of longitudinal (horizontal) magnetization. Thus, the amount of information stored in the special recording media can be increased accordingly by vertical magnetization.
The recording media provided on one or both sides with at least one magnetic recording layer must have an extremely smooth surface in order thus to avoid difficulties in guiding the magnetic heads over these surfaces, since the guidance height, also called flying height, of the heads is generally less than 1 .mu.m (see the above cited German patent). Consequently, also the disc-shaped disc carrier or an intermediate layer applied to it, on which then the at least one recording layer must be deposited, must have an accordingly small surface roughness. The carrier body or the intermediate layer is therefore generally polished to a correspondingly smooth mirror surface.
Accordingly, in the recording medium known from the cited German Pat. No. 29 23 682, its disc-shaped carrier body of nonmagnetic material such as an Al-alloy, designated as the base plate, is first subjected on at least one flat side to a facing-off and heat-smoothing process. Onto the so pre-treated carrier body is then plated an intermediate layer of a nonmagnetic material such as nickel-phosphorus with a thickness of about 50 .mu.m. This intermediate layer is subsequently finely processed mechanically by polishing to a mirror surface with a surface roughness of 0.04 .mu.m or less and a thickness of about 30 .mu.m. To this mirror surface are then applied several recording layers of magnetic metal, their axis of easy magnetization extending in the direction perpendicular to the surface of the recording medium. A special protective layer is finally deposited on the stratified structure so obtained. In the manufacture of this known recording medium, however, the process step of polishing the intermediate layer, in which special intermediate anneals must be performed, is relatively costly.
From DE-OS No. 30 14 718 is further known a multi-layer magnetic thin film disc as a recording medium, the disc-shaped carrier body of which consists of aluminum or an aluminum alloy and especially of the aluminum-magnesium alloy AlMg.sub.5. This carrier body is coated with a thin layer, for instance, 1 .mu.m thick, of a synthetic resin, to which at least one metallic magnetic layer is then applied as the recording layer. So that a magnetic head can be guided over this thin film disc at the required low flying height, also the carrier body of this disc must of necessity be subjected to a suitable polishing process which may in particular be a so-called gloss treatment. However, the danger exists that the relatively soft material of the AlMg alloy preferentially used is plastically deformed, i.e., that the disc-shaped carrier body becomes wavy.